Infestation of the body by lice is an age old problem. Reference to these pests can be found throughout documented history. Lice have been responsible for the spread of typhus, causing decimation of many armies and navies of the military powers of the 15th, 16th, 17th and 18th centuries. Lice are still considered as disease vectors and present serious health problems throughout the world. Not only do lice carry a wide variety of bacteria on their exterior surfaces, but their fecal matter transmits disease when it enters the puncture wounds lice inflict during feeding.
The human lice genus includes pubic lice, body lice, and head lice. Although related, each of them have specific characteristics with regard to habitat and feeding. The present invention is most useful in treatment of head lice. Head lice are small hard-shelled ectoparasites which cling to hair follicles while feeding, mating and laying eggs. The louse must remain on the head as it will die within a short period of time when removed. Head lice proliferate at an incredible rate. A louse is ready to mate and reproduce within 10 hours after hatching. Under ideal conditions, a female louse may produce up to 300 eggs in its lifetime. Ideal conditions include an adequate food supply, environmental temperatures from about 28.degree. C. to about 32.degree. C., and relative humidity from about 70% to about 90%. Poor hygienic and grooming habits are also known to contribute significantly to the spread of lice. Thus, lice infestations are most serious in tropical areas where the inhabitants have both substandard hygienic facilities and practices.
The louse's hard keratinous shell serves as protection from external elements. Lice eggs (or ova) are similarly protected by a chitinous sheath surrounding the eggs and attached to the hair follicles. Although the lice may be affected by the use of an insecticide, often the eggs remain resistant to attack. Thus, the optimum chemical treatment should include both a pediculicide, which kills the adult lice, and an ovicide, which interrupts the gestation of the eggs.
Biologically active agents for the control of lice are well known in the art. Lindane (gamma-benzene hexachloride), synergized natural pyrethrins, and synthetic derived compounds known as pyrethroids have all been used as pediculicides in lice treatment compositions. However, since lindane has a poor safety profile and lice have developed a significant degree of resistance to it, natural pyrethrins and synthetic pyrethroids are now routinely chosen for use in pediculicide and ovicide compositions.
Natural pyrethrins are made from extracts of naturally insecticidal chrysanthemum flowers and have been used since the early 1930's. European Patent Application 191,236 published Aug. 20, 1986; European Patent Application 262,885, published Apr. 6, 1988; and British Patent Specification 1,593,601, published Jul. 22, 1981, all disclose the use of natural pyrethrins for treating lice. U.S. Pat. No. 4,668,666, Allan, issued May 26, 1987, notes that natural pyrethrin necessitates frequent follow-up treatments because its poor environmental stability only provides short term residual action.
Synthetic pyrethroids became popular during World War II when chrysanthemum flowers became nearly impossible to get. Besides being available at lower prices, they were also somewhat more stable than the natural product.
The toxicity of both natural and synthetic pediculicides upon insects is described in Clements, May, and Pesti, The Actions of Pyrethroids upon the Peripheral Nervous System and Associated Organs in the Locust, 8 Pesticide Science 661-680 (1977).
Although generally more effective against lice than natural pediculicides, some of the synthetic actives are more toxic to the subject being treated. To reduce safety risks to the user antilice compositions are formulated with a combination of natural and synthetic pediculicides. The combination is thought to be the most effective since natural pyrethrins are known to affect certain nerve response mechanisms that synthetic pyrethroids cannot. U.S. Pat. No. 4,668,666, Allan, discloses anti-lice treatment compositions containing combinations of natural pyrethrins and synthetic pyrethroids from about 5:1 to about 1:5. These compositions, however, are relatively unstable and the actives must be encapsulated with aryl siloxane polymers to minimize destabilization by environmental elements. Thus, aryl siloxane polymers are an essential component in the Allan compositions in order to achieve some degree of compositional stability. However, even the use of siloxanes alone does not prevent the separation of the actives in hair treatment compositions such as shampoos, lotions and conditioners at temperatures over 38.degree. C. This is a particular problem in tropical areas where lice infestation is most prevalent. Once the actives separate, they cannot be reincorporated back into the lotion, shampoo or conditioner, thereby, negatively affecting the compositions, safety and effectiveness.